The present invention relates to methods and apparatus for monitoring the operation of sucker-rod well pumping units, and more particularly to methods and apparatus for detecting the breakage of a sucker-rod, and other pump problems in wells employing sucker-rod pumping units.
Sucker-rod type pumping units are widely used in the petroleum industry in order to recover fluid from wells extending into subterranean formations. Such units include a sucker-rod string which extends into the well and means at the surface for an up and down movement of the rod string in order to operate the plunger of a downhole pump. Typical of such units are the so called "beamtype" pumping units having the sucker-rod string suspended at the surface of the well from a structure consisting of a Samson post and a walking beam pivotally mounted on the Samson post. The sucker-rod string normally is connected at one end of the walking beam and the other end of the walking beam is connected to a prime mover such as a motor through a suitable crank and pitman connection. In this arrangement the walking beam and the sucker-rod string are driven in a reciprocal mode by the prime mover.
A variety of malfunctions such as worn pumps, sucker-rod part, split tubing, and stuck pump valves can interrupt the pumping of fluid from a well. Such malfunctions can be caused by normal wear and tear on the equipment, by the nature of the fluid being pumped or they could be caused by abnormal pumping conditions.
Rod part is the separation or breaking of sucker-rods in a rod pumped well due to such causes as corrosion, improper rod loading, and other types of rod damage. These failures may be either tensile failures or fatigue failures. When an applied load exceeds the tensile strength of the sucker-rod string a necked-down or reduction of the cross-sectional area can occur and the rod string will fail at the necked-down portion of the rod. This rare type of failure can occur when a pump is stuck and too much strain is placed on the rod string.
Fatigue failures occur in sucker-rod steel at applied loads less than the yield strength of the steel under cyclic load conditions. Failures can be caused by changes in fluid volume, in fluid level, by the corrosiveness of the fluid being pumped, by changes in the length or speed of the pumping stroke, or by other causes.
Mechanical damage to the sucker-rod can cause failure in either the rod joints or in the rod body. This damage can be due to the physical handling or makeup of the rod string or by motion of the rod string in the pumping cycle. The rod string is a long flexible member that should remain straight during the pumping cycle. Constant lateral or side movement of the rod string will induce stress fatigue cracks in the same manner as repeatedly bending a wire until it breaks. Bending can also cause the rod to fail due to rubbing against a casing string in the well.
The most detrimental cause of sucker-rod flexing is fluid pound. Fluid pound occurs when the well is pumped-off, i.e., when fluid is withdrawn from the well at a rate greater than the rate at which fluid enters the well from the formation. When this occurs, the working well of the downhole pump is only partially filled during an upstroke of the plunger and on the downstroke the plunger strikes or "pounds" the fluid in the working barrel causing severe jarring of the entire pumping unit. The jarring causes shock waves to travel throughout the sucker-rod string to produce flexing, overload, coupling and pin failures, and accelerates fatigue cracking from corrosion pitting and mechanical damage. Rod part can occur anywhere along the length of the sucker-rod string. When the parting occurs in the upper portion of the rod, rod part detection is relatively easy because of the change in weight of the rod connected to the walking beam, but deep well parting is difficult to detect because there may be relatively little change in rod weight.
The downhole pump includes a standing valve located at the lower end of the pump to allow fluid to move into the pump, and a plunger which moves up and down to move the fluid to the top of the well. A traveling valve located on the plunger allows fluid to move above the plunger on the downstroke of the plunger and allows the plunger to lift fluid toward the top of the well on the upstroke. If either of the valves should become stuck open or if the plunger should become stuck to the sidewalls of the pump upward fluid flow will stop and damage could quickly result to other portions of the pumping system.